Installation of a tubular body, such as a large pipe, in such facilities as nuclear power plants and large-scale plants has a problem of removing a stress remaining in the pipe after welding. Welding causes a residual stress in a pipe, and the residual stress may possibly shorten the life of the pipe. For this reason, it is desirable that such residual stress caused by welding be removed.
The inventors of the present application have proposed a method for improving the residual stress that remains in a pipe. The outer circumferential surface of a pipe is irradiated with laser light emitted from a laser that is moving around the pipe. The pipe is thus rapidly heated up to a temperature that will not negatively affect the material of the pipe. The heating causes a temperature difference between the inner surface and the outer surface of the pipe, and thus contributes to the reduction in the residual stress in the pipe (see patent Documents 1 to 4).
Patent Document 1: JP-A-2006-035292
Patent Document 2: JP-A-2006-015399
Patent Document 3: JP-A-2006-037199
Patent Document 4: JP-A-2005-232586